The Stovebolt Site Forums The Shop Area Engine Shop Overdrive for a SM420 4-speed?

Hey guys,
I know this topic has been discussed alot, I can't tell you how many posts I've read about overdrives since I got my truck last May and found out that the top speed was 45mph (3/4 ton w/ 4.57 r&p). But I can't find a post that documents these elusive bolt-on overdrives for the SM420s with anything more than a vauge refference to Truckstell or Brownie or a link to the $3,000 gear vendors unit, etc. In all of the other posts that I've read the topic always ends up in changing out the transmission or the rear end or ring and pinion. I'd like to try and compile information on these overdrive installations (it'd make a great tech tip...), so my question is: What overdrives will bolt up (or are easily adapted) to work with this transmission (modern and era appropriate (aka old ))? What vehicles can they be found on? Does anyone with these units have make/model numbers? Did they pull it off of another vehicle? Can the overdrives used on cars be used on our trucks? Are there some external overdrives that are welded into the torque tube? If so what makes/models and could they be made to work on the 3/4 ton's first enclosed drive shaft section? Finally, assuming you can find one, what resources are availible for rebuilding these units?
Thank you in advance for the advice/informaion. It would be great to combine all this widely strewn knowledge into one place, where those who want to keep their rock-solid SM420s can learn what to look for in their search for this holy grail. This is an amazing site by the way, y'all have saved my hide (or my truck's) on more than one occasion.
Thanks again and sorry if I tend to ramble,
-Alan

I have a Gear Vendors unit. As far as I know, it is the only thing out there that will bolt up to an sm420. It is worth every penny. I get to keep my granny gear and 3:90 posi, and can overdrive 2nd, 3rd and 4th. The split between 3rd and 4th is exactly what our trannys need. 4th over is more cake and icing. I can go 75 at 2500 rpm and can have a conversation or listen to the stock radio. It is good for a working/towing set up and when used in that manner it is much stronger and will outlast any s-10/camaro 5 speed. GearVendors.com

I've seen mentioned modifying the SM420 to use the Saginaw 3spd overdrive. I just did a search and couldn't find anything. Thought there was even a tech tip on how to do it.

Larry

The truckstell unit is one of those things that you may see one in a lifetime but unfortunately someone is using it in their truck. The BW OD incorporated into a torque tube was once a popular conversion, however those have dried up also. It is still possible to find a BW unit and have the machine work done but in my opinion it would be cost prohibitive. You would have additional issues being as your truck only has a partial torque tube. There are 3 speed units available for 55-69 cars & trucks which work fine but to use one you would give up your 4 speed. It would require linkage, driveshaft mods, etc. I think what Larry was referring to is using a BW OD unit from a 66-69 Saganaw 3 speed and attaching it to a passenger car 4 speed. There should be some info on that in this forum. You could also replace the entire rear end with something from a later model truck, however it would require some modifications also.
Finding the correct width & gear ratio would also be a challenge.
You would probably want something with a ratio in the mid 3's to make much of a difference.
Fred

Fred, I think your right it was the BW OD and a Saginaw. 8-[

Larry

There is another option to the Gear Venders overdrive, the Ranger overdrive from Advance Adapters. It mounts in front of the trans, moves it back about 7". It's lever actuated instead of cable/electric. Last time I checked, they ran around $1200 for the one that bolts up to an SM420.

What about an SM460 equipped with a transfer case for 4WD? Am I out of luck on that combo?